Recently, the development of electronic paper has been promoted in the industrial field, an educational foundation and the like. As application fields where electronic paper can be used, there are an electronic book, the monitor display apparatus of a mobile terminal set, etc., the display unit of an IC card, etc., and the like and various application forms are proposed and developed in each field. Furthermore, recently, newspaper information has been distributed on the Internet and electronic paper has been focused as an information medium instead of the conventional newspaper.
One leading method of electronic paper is a method using a cohlesteric liquid crystal and this uses the superior features of a cohlesteric liquid crystal, that is, characteristics of keeping semi-permanent display (memory-property), vivid color display, high contrast and high resolution.
Since the molecule of a cohlesteric liquid crystal forms a helical cohlesteric phase by adding fairly much (several-tens percentage of) chiral additive (chiral material) to a cohlesteric liquid crystal, such a cohlesteric liquid crystal is also called chiral nematic liquid crystal.
FIGS. 1A and 1B illustrate the state of a cohlesteric liquid crystal. As illustrated in FIGS. 1A and 1B, a display element 10 using a cohlesteric liquid crystal includes a top-side substrate 11, a cohlesteric liquid crystal layer 12 and a bottom-side substrate 13. The operational state of a cohlesteric liquid crystal includes a planer state capable of reflecting incident light as illustrated in FIG. 1A and a focal-conic state capable of transmitting incident light as illustrated in FIG. 1B. Both these states are maintained in a state where no voltage is applied, that is, under no electric field. Therefore, a cohlesteric liquid crystal can hold a stable display state.
When the operational state of a cohlesteric liquid crystal is a planer state, light of a wavelength corresponding to the helical pitch of the liquid crystal molecule is reflected. A wavelength λ in which reflection becomes large can be expressed to be n·p (λ=n·p) assuming that the average refractive index of a cohlesteric liquid crystal and its helical pitch are n and p, respectively.
Meanwhile, characteristically the reflection band Δλ of a cohlesteric liquid crystal widely varies depending on the refractive index anisotropy Δn of the liquid crystal.
When the operational state of a cohlesteric liquid crystal is a planer state, it becomes a “light” state because of reflection of incident light, that is, a state capable of displaying white. Meanwhile, when the operational state of a cohlesteric liquid crystal is a focal-conic state, it becomes a “dark” state, that is, a state capable of displaying black. That is because when a light absorptive layer is provided under the bottom-side substrate 13, light transmits through a liquid crystal layer and also it is absorbed by the light absorptive layer.
The driving method of a conventional general display element using a cohlesteric liquid crystal will be explained below.
FIG. 2 is a graph illustrating the voltage-reflectance characteristic of a conventional general cohlesteric liquid crystal.
In the graph illustrated in FIG. 2, the vertical and horizontal axes of the graph indicate the reflectance (%) of a cohlesteric liquid crystal and the voltage value (V) of a pulse voltage applied to between electrodes pinching a cohlesteric liquid crystal with a predetermined pulse width, respectively.
A curve P indicated by a solid line indicates the voltage-reflectance characteristic of a cohlesteric liquid crystal whose initial state is a planer state and a curve FC indicated by a broken line indicates the voltage-reflectance characteristic of a cohlesteric liquid crystal whose initial state is a focal-conic state where incident light is transmitted.
When a relatively intense electric field is generated in the cohlesteric liquid crystal by applying a predetermined high voltage VP100 (for example, ±36V) to between electrodes pinching the cohlesteric liquid crystal, the helical structure of the cohlesteric liquid crystal is completely released and it moves to a homeotropical state where all molecules follow the direction of the electric field.
When the electric field in the cohlesteric liquid crystal is suddenly reduced to almost zero by suddenly reducing an applied voltage from VP100 to a predetermined low voltage (for example, VF0=±4V) while the molecules of the crystal liquid is in a homeotropical state, the helical axis of the cohlesteric liquid crystal becomes perpendicular to the electrode and transits to a planer state where light corresponding to the helical pitch is selectively reflected.
Meanwhile, a relatively weak electric field is generated in the cohlesteric liquid crystal by applying a predetermined low voltage VF100b (for example, ±24V), it enters a state where the helical structure of the cohlesteric liquid crystal molecule is not completely released. When the electric field in the liquid crystal is suddenly reduced to almost zero by suddenly reducing the applied voltage from VF100b to low voltage VF0 in this state or when the electric field is slowly eliminated by applying an intense electric field, the helical axis of the liquid crystal molecule becomes parallel to the electrode, namely, it enters the above-described focal-conic state where the incident light is transmitted.
When the electric field is suddenly eliminated by applying an intermediately intense electric field, gradation display becomes possible since the above-described planer state where the incident light is reflected and the above-described focal-conic state where the incident light is transmitted are mixed. Conventionally, a liquid crystal display apparatus displays images by using reflective and absorptive functions of the incident light, as described above.
The principle of the driving method based on the above-described voltage response characteristic will be explained in more detail with reference to FIGS. 3A through 3C.
FIG. 3A illustrates a pulse response characteristic in the case where the pulse width of a voltage pulse is several tens ms in the cohlesteric liquid crystal, FIG. 3B illustrates a pulse response characteristic in the case where the pulse width of a voltage pulse is 2 ms and FIG. 3C illustrates a pulse response characteristic in the case where the pulse width of a voltage pulse is 1 ms in the cohlesteric liquid crystal. A voltage pulse applied to the cohlesteric liquid crystal is indicated on the top-side of each of FIGS. 3A through 3C and a voltage-reflectance characteristic on the bottom side. The vertical and horizontal axes of FIGS. 3A through 3C indicate a reflectance (%) and a voltage (V), respectively. For the drive pulse of the cohlesteric liquid crystal, a combination of positive and negative pulses is used. As well known, when a fixed pulse whose polarity is not inverted continues to be applied to the cohlesteric liquid crystal, the degradation of the cohlesteric liquid crystal, due to polarization is induced. However, such degradation can be prevented by using a combination of positive and negative pulses.
In FIG. 3A, when the pulse width of a voltage pulse applied to the cohlesteric liquid crystal is as large as several tens ms, in the case where the initial state is a planer state, it enters a focal-conic state when the voltage is increased to a certain level, as illustrated by a solid line, and it returns to a plenary state when the voltage is further increased. However, as illustrated by a broken line, in the case where the initial state is a planer state, it gradually transits to a planer state as the pulse voltage is increased.
When the pulse width of a voltage applied to the cohlesteric liquid crystal is large, the pulse voltage in which it always enters a planer state regardless of whether it is either a planer or focal-conic state is ±36V in FIG. 3A. When an intermediate pulse voltage is applied, gradation display can be obtained since planer and focal-conic states are mixed in the cohlesteric liquid crystal.
Meanwhile, when the pulse width of a voltage pulse applied to the cohlesteric liquid crystal is as small as 2 ms, as illustrated in FIG. 3B, in the case where the initial state is a planer state, the reflectance does not change when the pulse voltage is 10V. Since planer and focal-conic states are mixed when the pulse voltage is more than 10V, the reflectance degrades. This amount of degradation of the reflectance increases as the applied voltage increases. However, when the applied voltage becomes more than 36V, the amount of degradation of the reflectance becomes constant. Such a characteristic in the cohlesteric liquid crystal also applies to a state where planer and focal-conic states are mixed in the initial state. Therefore, when in the case where the initial state is a planer state, the pulse width is 2 ms and the voltage pulse whose pulse voltage is 20V is applied once, the reflective index degrades somewhat. Therefore, in a state where planer and focal-conic states are mixed (that is, a state where the reflectance degrades somewhat), the pulse width of the voltage pulse is 2 ms and also the reflectance of the cohlesteric liquid crystal can be further degraded by further applying the voltage pulse whose pulse voltage is 20V. The reflectance can be degraded to a predetermined value by repeating the sequence of the above operations.
As illustrated in FIG. 3C, when the pulse width further decreases to 1 ms, as in the case where the pulse width is 2 ms, the reflectance of the cohlesteric liquid crystal can be further degraded by further applying the voltage pulse to the cohlesteric liquid crystal. In this case, the degradation rate of the reflectance becomes smaller than that in the case where the pulse width is 2 ms.
Judging from the above, if a pulse of 36V is applied with a pulse width of several tens ms, the cohlesteric liquid crystal enters a planer state. If a pulse of between ten several V and 20V is applied, it enters a state where planer and focal-conic states are mixed and the reflectance degrades. This amount of degradation of the reflectance relates to the accumulation time of the pulse.
Currently, various driving method for realizing multi-gradation display using the cohlesteric liquid crystal are proposed and developed. These can be roughly classified into two of a dynamic driving method (for example, see document 1) and a conventional driving method (see Non-patent document 1).
Since the drive waveform of the dynamic driving method is complex, the dynamic driving method requires a complex control circuit and a driver IC and also requires a low-resistance transparent panel electrode. Therefore, the manufacturing cost becomes high. Furthermore, the power consumption is also large.
Non-patent document 1 discloses the conventional driving method of gradually driving the cohlesteric liquid crystal from a planer state to a focal-conic state or from a focal-conic state to a planer state, at the fairly high speed of a semi-moving image rate by adjusting the application times of a short voltage pulse, using an accumulation time peculiar to the cohlesteric liquid crystal.
In the driving method disclosed in Non-patent document 1, since the driving speed is at the high speed of a semi-moving image rate, the driving voltage is set to 50 through 70V. Therefore, the cost of the circuit becomes high. Furthermore, in the “two phase cumulative drive scheme” described in Non-patent document 1, accumulation times in two ways of an accumulation time to a planer state and an accumulation time to a focal-conic state are used by using two stages of a “preparation phase” and a “selection phase”. Therefore, the display quality of display images cannot be improved. Furthermore, since a fine voltage pulse is frequently applied, the power consumption of the driver circuit becomes large.
Patent documents 2 and 3 disclose a fast-forward mode driving method based on the reset to a focal-conic state. In this driving method, fairly high contrast can be obtained compared with the above-described driving method. However, in the case of a general-purpose STN driver IC, since writing after the reset requires a supply-difficult high voltage and also becomes cumulative writing in which it is transited in the direction of a planer state, cross-talk to a semi-selected/non-selected pixel becomes a problem. Besides, since a fine pulse is frequently applied in this driving method too, the power consumption becomes large.
When gradation is set using an accumulation time in the conventional driving method, the differentiation of a pulse width is also possible in addition to the adjustment of application times of a short pulse as described above. Thus, the differentiation of a pulse width is effective in suppressing the power consumption than the adjustment of application times of a short pulse. In the following explanation, a method for and differentiating a pulse width and setting gradation by changing an accumulation time is called PWM (pulse width modulation).
Patent document 4 discloses the circuit composition of a method for applying positive and negative pulses, whose pulse widths are different, to a liquid crystal display as a pulse voltage although no cohlesteric liquid crystal is used.
Each of FIGS. 4A through 4C illustrates one example of a voltage pulse whose width is different disclosed in Patent document 4. In these examples, the pulse width is made longer in the descending order of FIGS. 4A, 4B and 4C.
The voltage pulses illustrated in FIGS. 4A through 4C have positive and negative pulses whose per unit pulse length are the same and whose widths are different. The degradation due to the polarization of the cohlesteric liquid crystal can be prevented by applying such a polarity-conversion voltage pulse.
As described above, as methods for differentiating gradation by differentiating the application cumulative time of a voltage pulse applied to the cohlesteric liquid crystal, a method for differentiating the application times of a short voltage pulse and a method for differentiating the width of an applied voltage pulse (PWM method) are well known.
In the method differentiating gradation by differentiating the application cumulative time of a voltage pulse applied to the cohlesteric liquid crystal, voltages as illustrated in FIGS. 3B and 3C are applied. In the method for differentiating the application times of a short voltage pulse, a voltage as illustrated in FIG. 5 is applied to a pixel.
In the cohlesteric liquid crystal, when a large voltage is applied, the state changes regardless of the polarity of the applied voltage. In the liquid crystal display apparatus using the cohlesteric liquid crystal, a scan line extending in the horizontal direction is written one by one and the shifting operation of a written scan line is repeated. Therefore, a voltage at a ground level and an intermediate voltage (for example, 15V) are applied to a selected scan line and other non-selected scan lines, respectively. Meanwhile, although a pulse of a large voltage (20V) is applied to a data line extending in the vertical direction. In this case, if the potential of parts other than the pulse width is assumed to be ground potential (GND), a large voltage in inverse polarity (−15V) is applied to a pixel in the non-selected scan line and the state of the cohlesteric liquid crystal changes.
In order to prevent such a state change of the liquid crystal, in the case of a liquid crystal display apparatus using the cohlesteric liquid crystal, as illustrated in FIG. 5, a base voltage of +10V and a pulse voltage of +20V are used in a positive-polar phase, and a base voltage of −10V and a pulse voltage of −20V are used in a negative phase. Thus, either +5V or −5V is applied to the pixel of a non-selected scan line and there is no change in the state of the liquid crystal. In a selected scan line, either +20V or −20V is applied to a pulse part and either +10V or −10V is applied to a base part other than it.
Furthermore, Patent document 5 intends to realize a liquid crystal display circuit capable of supporting various types and forms of liquid crystal display panels and discloses a liquid crystal display circuit including a plurality of segment/common switching circuit composed of a first switching circuit for switching according to a common setting signal between a start signal for setting a common signal by shifting one pulse and storage data for switching over to either “common” or “segment”, a flip-flop circuit operated by the output of this first switching circuit, a reset pulse signal and a common clock and a second switching circuit for switching the output of this flip-flop circuit by the above-described common setting signal.    Patent document 1: Japanese Laid-open Patent Publication No. 2001-228459    Patent document 2: Japanese Laid-open Patent Publication No. 2000-147466    Patent document 3: Japanese Laid-open Patent Publication No. 2000-171837    Patent document 4: Japanese Laid-open Patent Publication No. H4-62516    Patent document 5: Japanese Laid-open Patent Publication No. H11-38941    Non-patent document 1: Y. M. Zhu, D. K. Yang, “Cumulative Drive Schemes for Bistable Reflective Cohlesteric LCDs.”, SID 98 DIGEST, pp 781-801 (1998)
The display apparatus including the above-described passive matrix display element transfers and outputs data when the operation mode of the driver is in a segment mode. Then, by changing the operation mode of the driver to a common mode at once, it outputs the data transferred in the segment mode in a common mode. Furthermore, since the data is transferred when the operation mode of the driver is a segment mode and during this period the data is not outputted as in a common mode, the output of the driver is switched off in a segment mode. In such a driving method, since only data is transferred at the time of data transfer in a segment mode and the liquid crystal is not driven, it affects the response speed of the liquid crystal.
In the present invention, it is a problem to shorten this data transfer time and to improve the response speed of the liquid crystal.
This problem will be explained in more detail below.
FIG. 12 is a time chart illustrating the sequence of the output signal of a general passive matrix driver.
In FIG. 12, a pulse signal XCLK indicates a clock for retrieving data (see FIG. 6). A pulse signal LP indicates a latch pulse for data confirmation. A frame signal FR repeating cyclic rise and fall indicates a pulse polarity conversion control signal for recovering time-varying degradation peculiar to liquid crystal by inverting the polarity of the applied voltage. A switching signal S/C indicates a signal for switching over between segment and common modes. A display apparatus driving signal /DSPOF (DSPOF bar) is the drive signal of a liquid crystal display apparatus and more particularly it indicates the inverse signal of a compulsory off signal of the applied voltage (signal for switching off the applied voltage, that is, the signal DSPOF illustrated in FIG. 6). Furthermore, an OUT voltage is a voltage applied to the liquid crystal display in order to output (display) line data.
As illustrated in FIG. 12, when the switching signal S/C is on the segment side and the driver is in the segment mode, the conventional liquid crystal display apparatus (display apparatus including a passive matrix display element) transfers and outputs data. Then, when the switching signal S/C is switched over to the common side and the operation mode of the driver is instantaneously changed to the common mode, the data transferred to the liquid crystal in the segment mode is outputted (displayed) in the common mode. This output (display) is performed by applying an OUT voltage to the liquid crystal. Since when the driver is in the segment mode data is transferred and during this period data is not outputted as in the common mode, the output of the driver is compulsorily stopped by switching off the display apparatus driving signal /DSPOF (DSPOF bar) in the segment mode. As described above, in such a driving method, only data transfer is performed at the time of data transfer in the segment mode and the liquid crystal is not driven, thereby affecting the response speed of the liquid crystal. Such control for compulsorily stopping the output of the driver by switching off the display apparatus driving signal /DSPOF (DSPOF bar) also applies to at the falling time of the frame signal FR for inverting the polarity of an applied voltage. Since at this moment large current accompanies, by compulsorily stopping the output of the driver, rush current is prevented and voltage drop is suppressed.